Fibroblast growth factor receptors are widely expressed on breast cancer cells and we report a preliminary study to determine whether these could be useful as potential targets for delivery of a cytotoxic fibroblast growth factor 2-saporin conjugate. We show that this mitotoxin conjugate can displace I-125-fibroblast growth factor 2 binding though with reduced affinity compared to unlabeled fibroblast growth factor 2. For 4 out of 5 cell lines it is an effective inhibitor of cell growth, and cytotoxic for at least 2 of the lines. Inhibitory effects did not depend on responsiveness of cells to fibroblast growth factor 2. This activity was not achieved with free saporin. There may be potential uses for this conjugate in both experimental systems to study receptor function and subsequent processing, and also in clinical settings to eliminate breast cancer cells.